The present invention relates generally to paper coating compositions which comprise as major components therein one or more paper coating pigments and one or more of certain alkali-sensitive copolymer latexes as a binder component for such pigment. More particularly, the present invention relates to paper coating compositions comprising a pigment and an alkali-sensitive latex wherein said latex is a synthetic organic copolymer latex prepared by emulsion copolymerizing (A) from about 10 to about 90 parts by weight of a monomer charge comprising (1) from about 10 to about 60 weight percent of an ethylenically unsaturated carboxylic acid and (2) from about 40 to about 90 weight percent of a copolymerizable monoolefin compound in the presence of (B) from about 10 to about 90 parts by weight of the polymer solids of a copolymer latex comprising a monovinylidene aromatic monomer such as styrene and an open-chain aliphatic conjugated diene such as butadiene.
As is well known in the art, carboxylated styrene-butadiene copolymer latexes are useful and generally well suited for use as a binder component for the pigments in conventional paper coating compositions. Moreover, it is also known that such latex is typically not used as the sole binder component in such compositions but instead is generally used in conjunction with natural aqueous pastes such as casein, starch, and the like, in order to improve certain specific properties of the resultant compositions and/or of the ultimate coated paper articles made therefrom. Unfortunately, however, the use of the aforementioned natural aqueous pastes have certain drawbacks associated therewith such as difficulties in controlling the overall properties of the resulting compositions, decomposition of the compositions containing such pastes, and the like. Accordingly, various attempts have been made in recent years to provide coating compositions having the desired combination of properties without requiring the use of the aforementioned natural aqueous pastes and, as a result, certain approaches for achieving that objective have heretofore been proposed.
Typical examples of the aforementioned approaches include (a) those disclosed in U.S. Pat. Nos. 3,081,198; 3,513,121; and West German Patent No. 1,919,379 (i.e., U.S. Pat. No. 3,694,394) wherein an aqueous dispersion is employed which is a blend of (1) a generally alkali-insensitive polymer latex such as, for example, an alkali-insensitive styrene-butadiene copolymer latex and (2) an alkali-sensitive copolymer latex prepared by employing an ethylenically unsaturated comonomer containing a carboxyl group and (b) those disclosed in U.S. Pat. Nos. 3,409,569 and 3,575,913 and Japanese Published Examined Patent Application No. 49-44948 wherein certain carboxylated copolymer latexes such as, for example, copolymers of styrene, butadiene and an ethylenically unsaturated carboxylic acid are employed as a binder component to provide alkaline paper coating grade compositions which can be free of natural pastes such as casein, starch, etc.
While the aforementioned prior art methods appear to provide paper coating compositions and coated paper products which have somewhat improved properties relative to their prior art casein and/or starch-containing counterparts, such methods nevertheless still generally suffer from one or more drawbacks such as limited or unsatisfactory water resistance, undesirable viscosity sensitivity to factors such as changes in the composition pH, insufficient viscosity at low shear rates, inadequate fluidity at high shear rates, and the like. Accordingly, it would be highly desirable to provide improved aqueous paper coating compositions which eliminate (or which at least substantially reduce the extent of) the foregoing difficulties which have heretofore been encountered with the various above-described prior art paper coating compositions.